Stop the Sentinels

Stop the Sentinels! is an event in Chapter 5, specifically in the second area, the Ruined Projects. When triggered, this event is dangerous for everyone. Sentinels will appear in random areas of the map, which are somewhat strong to begin with. They go down after about a dozen hits, so they're somewhat stronger than minions, but a far cry from bosses.

However, after eight sentinels are destroyed, a Mega Sentinel will arrive somewhere on the map. This is by far the strongest, most difficult event boss you've had to deal with thusfar.

The Mega Sentinel has a variety of attacks to cause many Heroes problems, but he's especially dangerous against melee characters. Although the Mega Sentinel has ranged attacks, he may use them against any melee characters at zero range!

His melee attacks are a series of kicks and a ground stomp. Both can be used in a combo, and can quickly decimate a hero's health; if a ranged fighter gets caught in it, they may be taken down instantly off just a single combo. If you're melee, you'll want to back off if you have less than three-quarters health to heal up, just to be on the safe side.

Mega Sentinel's ranged attacks are a blue eye laser for moderate damage, or a flaming eye blast for serious damage. The latter also produces a cloud of fire that will continually damage anyone who stands in it. Still, both ranged attacks have somewhat lower range than most heroes' ranged attacks. Although the blue laser comes out instantly, the flaming blast has a slower trajectory and could be avoided if you keep moving.

Mega Sentinel has battlefield awareness. If any hero attempts to revive another, the Mega Sentinel will usually turn his aggro immediately on whoever is doing the reviving, thus interrupting it. As a melee fighter, you should be doing most of the reviving if you've had to back off to heal, but be aware not to attempt the revival process until you have some health to spare.

The ranged fighters will be doing most of the damage here, and if you have both types of moves (such as Deadpool), stick with your ranged. This will keep you out of range of his most devastating attacks, and if you keep moving, you should be able to dodge most of his ranged attacks as well. Still, the melee fighters have their role, because if you don't have enough tanks fighting him, he'll be free to chase down the ranged fighters. Either way, you'll need a good dozen players at least to deal with him, including at least eight ranged fighters, or he'll be able to keep you down with his attacks.

If you take him down, you'll be rewarded by a S.H.I.E.L.D. agent who stands near the Ruined Projects waypoint.